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Mysterious Tremors' Strength Ebbs With Tides

John Roach
for National Geographic News
November 22, 2007
 
The intensities of strange, long-lasting tremors in North America's Pacific Northwest ramp up and quiet down with the rise and fall of the ocean's tides, according to a new study.

These so-called nonvolcanic tremors are very faint seismic signals that were not discovered until 2002. Their exact cause remains a mystery.

Scientists are almost certain that the tremors are not earthquakes. Unlike quakes, the tremors start very deep in Earth's crust and can last for weeks at a time.

Although the new study does not reveal how the tremors are triggered, it does suggest that the tides—forces controlled by the gravitational tugs of the sun and the moon—play a key role in how intensely they vibrate.

"When the water level is up from tides, we see that the tremor is about 30 percent stronger than average," said study co-author Justin Rubinstein, a seismologist at the University of Washington in Seattle.

"And when the tides are out, it's about 30 percent weaker than average."

Rubinstein and colleagues present their data this week in the online version of the journal Science.

Earth's Pulse

Almost all nonvolcanic tremors have been detected on so-called subduction faults, where one piece of Earth's crust slips beneath another.

The have been picked up in the Cascadia region of the Pacific Northwest, which stretches from Vancouver Island in Canada to northern California in the U.S. (see a map of the region).

They have also been found occurring in the subduction zone along southwest Japan's Nankai trough.

In 2004 researchers detected nonvolcanic tremors deep in California's San Andreas Fault—the first time they had been found occurring on a strike-slip fault, where Earth's plates slide past each other.

Previous studies suggested that the tremors could be signals of impending earthquakes, offering hope that they could be used to better predict the natural disasters.

To get a clearer picture of what makes these tremors work, Rubinstein's team deployed arrays of seismic sensors off the coast of Cascadia and collected data between 2004 and 2007.

Their results showed a distinct pulsing of the tremors that matched with tidal motions.

But the researchers are quick to point out that their findings do not support a link between the moon's gravity and earthquakes—a theory that has been pursued by researchers since the 1800s.

Instead the team hypothesizes that the tremors occur on very weak faults that are easily influenced by slight gravitational tugs.

Gregory Beroza is a seismologist at Stanford University in California who studies nonvolcanic tremors.

He agreed that the tremors' response to the tides suggests that weak bonds hold the faults together.

"It doesn't take much to perturb this system, because if it did, there wouldn't be this correspondence between the tremor and the Earth tides," Beroza said.

Tremor Triggers

Rubinstein also co-authored a related study online in this week's Science that expands the criteria for where nonvolcanic tremors are likely to occur.

This study, led by Joan Gomberg at the U.S. Geological Survey in Seattle, Washington, reports that a magnitude 7.8 earthquake in Alaska in 2002 triggered tremors in seven locations on strike-slip faults in California.

"We show that [a nonvolcanic tremor] is a much more broadly distributed phenomenon than we thought," Rubinstein said.

That throws a wrench in scientific efforts to determine the criteria necessary to trigger the tremors.

Except for the tremors along the San Andreas, the vibrations were believed to be common to subduction zones.

Researchers had therefore suspected that the tremors required conditions unique to those areas: depth, high pressures, high temperatures, and fluids.

But the California strike-slip faults examined for the new study are at moderate depth, with lower pressure and temperature and probably lower fluid levels.

"These criteria we all thought were necessary for tremors don't seem to be the right criteria," Rubinstein said.

David Shelly is a research fellow at the University of California, Berkeley, who studies nonvolcanic tremors but was not involved in either of the new studies.

Both findings demonstrate that "it doesn't take a very big change to induce tremor activity," he said.

According to Stanford's Beroza, the new findings increase scientific understanding of the tremors and may one day allow for better analysis of earthquake hazards.

"We can't say yet until we figure out exactly what's going on how [the tremors] might inform seismic hazard," he noted.

"But I will say that the tremor in Cascadia and Japan—and apparently California—occurs on the deep extension of the very same faults that have the kind of earthquakes that everyone is scared of."

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